Roadholding, driving characteristics and comfort are important parameters of motor vehicles such as cars, military vehicles, trucks and the like. In order to improve roadholding, driving characteristics and comfort vehicles are equipped with suspension systems, which are arranged to protect the vehicle, passengers and load from damages and wear. A suspension system comprises spring members for maintaining the vehicle on the right level and absorb impacts such that irregularities in the ground do not propagate to the vehicle body, and damping members for extinguishing the suspension movement of the vehicle body. The damping member also contributes to converting kinetic energy of the vehicle body to heat for preventing impact of the suspension system, particularly of terrain vehicles.
In so called passive damping systems the damping when for example jumping with the vehicle in a ramp-like situation is normally too low, which results in the vehicle by impact is strongly pressed together, which results in great stresses on the vehicle and low comfort. This may with a passive damping system be solved by increasing the damping of the damping system. This however results in the disadvantage that the damping becomes too high during normal drive on grounds with small irregularities, which results in low comfort.
Lately more active damping systems adapting to the current state of the vehicle have been developed. A so called semi-active damping system is, by means of some fixed restrictions, arranged to regulate the damping while the spring has a non-variable spring constant. An active damping system is arranged to regulate the damping infinitely based on the movement of the vehicle. Active damping systems may also replace stabilizers in cases where the damping torque may be regulated independently of the difference in speed between wheels and body.
JP2772155 describes a hydropneumatic damping system of a tracked vehicle wherein the damping is arranged to be controlled between two passive positions depending on vertical speed/speed difference between chassis and wheels. The damping is provided by choking an oil flow. During jumping with the vehicle in ramp-like situation this damping system results in an improved performance and comfort in that during landing the damping is controlled to a passive position with great damping while during normal drive the damping is controlled to a passive position with little damping. However, the damping system according to JP2772155 does not take for example degree of impact into consideration whereby performance only is optimized for two situations, which in certain cases may lead to bad performance and comfort.
U.S. Pat. No. 5,071,157 describes a system for controlling the suspension of a vehicle where the system measures the vertical movement of the wheels and the vertical movement of the vehicle in the corners of the vehicle. From measured values different kinds of movements of the vehicle may be detected (“heave”, “roll” and “pitch”), whereupon the damping may be controlled based on the detected vehicle movements. The vehicle is provided with dampers between wheels and chassis, where the damping of the dampers may be regulated infinitely with respect to the measured values. Further measured data, such as data regarding steering angle and braking, may also be used to regulate the damping.
US20040231904 describes an active suspension system for a wheeled vehicle where the wheels are suspended by means of resilient rocker arms which selectively may be locked with a rotation damper of the respective rocker arm shaft, which is fixed to the rocker arms. Control of the respective rotation damper may be effected based upon a plurality of different signals from a torque or position sensor of the rocker arm or rocker arm shaft, etc., and aiming at obtaining the best possible capability.
The active or semi-active damping described in these and other documents increases the comfort during drive of the vehicle. There is however a need to further increase the comfort during drive of motor vehicles, and particularly there is a need of a suspension system increasing the comfort during heavy braking of a tracked vehicle.